It is likely that complexed forms of Na ion, K ion, Mg ions and Ca ions are of importance in partially regulating many cellular functions, including transmembrane transport. None of the techniques thus far applied to the problem are sufficiently sensitive to detect these complexes directly. The recent development of double resonance methods has enormously increased the sensitivity of nuclear quadrupole resonance (NQR) spectroscopy. We propose to further develop these techniques in order to search for intracellular and intramembrane complexes of Na ion, K ion, Mg ions and Ca ions, to define these complexes and to relate them to physiologic events, particularly to ion transport. A double resonance spectrometer has been assembled and NQR of simple salts and complexes of these ions are being investigated.